1. Field of the Invention
The present invention relates to a method for manufacturing a semiconductor device with transistors, each of which has several independent size of channel length, and each of which is inhibited short channel effect moderately.
This is counterpart of and claims priority to Japanese application Serial Number 418282/2003, filed on Dec. 16, 2003, the subject matter of which is incorporated herein by reference.
2. Description of the Related Art
It is known that transistor's threshold level of gate voltage changes when the transistor's channel length is shorted. The phenomenon is called “short channel effect”. The short channel effect can, in part, be inhibited by making a doped layer, which is called “pocket layer”, under a channel layer of the transistor.
By the way, a substrate, which is called silicon-on-insulator (SOI) substrate, is used for forming transistors on the substrate, in order to enhance the transistor speed and reduce the transistor's power consumption. An SOI substrate is a substrate in which a insulator layer is buried.
To inhibit short channel effect of the transistor formed on the SOI substrate, a pocket layer is formed in region deeper than the insulator layer under the transistor. The electric field, which occurs between the pocket layer and the gate electrode, decreases in a condition where the pocket layer is formed under the insulator layer, compared to a condition where the pocket layer is formed over the insulator layer. Therefore, carrier mobility increases, and transistor speed is enhanced.
Whether a bulk substrate is used or an SOI substrate is used to form a transistor, the more impurity density of the pocket layer increase, the more short channel effect of the transistor is inhibited effectively. But, electric field, which occurs between the pocket layer and the gate electrode, is enhanced. Therefore, carrier mobility decreases. On that account, the transistor speed decreases. To prevent transistor speed decreasing, the pocket layer is shrunk to be formed only the region near a source/drain of the transistor. However, if the pocket layer is too shrunk the inhibition of short channel effect is not enough.
Therefore, the size of the pocket layer must be controlled to strike a balance between inhibition of short channel effect and prevention of decrease of carrier mobility. To strike a balance between inhibition of short channel effect and prevention of decrease of carrier mobility, the size of the pocket layer is matched for the channel length of each transistor. Concretely speaking, the longer channel length of the transistor is, the more high density pocket layer is matched. Therefore, if forming transistors with several independent size, and forming pocket layers which have same size for each transistors, a part of the transistors have pocket layers with inappropriate size.